Greenwich Mean Time (GMT) began in 1675. This was when the Royal Observatory at Greenwich was built to help ships find their longitude at sea. GMT was a standard reference for timekeeping when each city in England kept a different local time. By about 1900, almost all time on earth was in the form of standard time zones. Now, all places on earth are divided into time zones.

Greenwich Mean Time is now called UTC (Coordinated Universal Time). UTC is the time standard of the world. All other parts of the world are offset (plus or minus) according to their longitude. Most of the zones are offset by a full hour, but there are some offset by half an hour or 45 minutes.

Standard World Time Zones map (as of August 2015)

Images

Plaque commemorating the Railway General Time Convention of 1883 in North America

1913 time zone map of the United States, showing boundaries very different from today

Political map showing how a great part of the world has a gap between the official time and the solar.